SSLs use semiconductor light-emitting diodes (“LEDs”), polymer light-emitting diodes (“PLEDs”), organic light-emitting diodes (“OLEDs”), or other types of SSEs as sources of illumination. Generally, SSLs generate less heat, provide greater resistance to shock and vibration, and have longer life spans than conventional lighting devices that use filaments, plasma, or gas as sources of illumination (e.g., florescent or incandescent lights).
A conventional type of SSL is a “white light” SSE. White light requires a mixture of wavelengths to be perceived by human eyes. However, SSEs typically only emit light at one particular wavelength (e.g., blue light), so SSEs must be modified to emulate white light. One conventional technique for modulating the light from SSEs includes depositing a converter material (e.g., phosphor) on the SSE. For example, FIG. 1A shows a conventional SSL 10 that includes a support 2, an SSE 4 attached to the support 2, and a converter material 6 on the SSE 4. The SSE 4 can include one or more light emitting components. The SSE 4 typically emits blue light that stimulates the converter material 6 to emit light at a desired frequency (e.g., yellow light). The combination of the emissions from the SSE 4 and the converter material 6 appears white to human eyes if the wavelengths of the emissions are matched appropriately.
FIG. 1B shows a conventional structure for the SSE 4 that includes a silicon substrate 12, an N-type gallium nitride (“GaN”) material 14, an indium gallium nitride (“InGaN”) material 16 (and/or GaN multiple quantum wells), and a P-type GaN material 18 on one another in series. The SSE 4 shown in FIG. 1B is a lateral-type device that includes a first contact 20 on the P-type GaN material 18 and a second contact 22 on the N-type GaN material 14 spaced laterally apart from the first contact 20.
One challenge of conventional SSLs is that some of the components are sensitive to heat. Although SSLs produce less heat than conventional lighting devices, the heat generated by the SSEs causes such heat sensitive components to deteriorate and fail over time. For example, the phosphor and the junctions deteriorate at a faster rate at higher temperatures than at lower temperatures. The deterioration of the phosphor causes the light to change color over time, and the deterioration of the junctions reduces the light output at a given current (i.e., reduces the efficiency) and the life span of the device. Therefore, it is desirable to control the heat in the SSLs to maintain their color and intensity over a long life span.
Many SSL designs control the heat in the device by drawing heat away from the SSEs. Although this is a useful approach, it may not be possible to remove an adequate amount of heat to protect the device. Another approach is to limit the amount of heat generated by the SSEs. One conventional technique for limiting the heat generated by the SSEs is to provide an active temperature control system having a temperature sensor located in the SSL and a feedback controller that modulates the drive current to the SSE based on the signals from the temperature sensor. Such active temperature control systems increase the complexity of the SSLs and can lead to higher costs. Another existing technique for limiting the heat generated by the SSEs is to passively limit the current through the SSEs. For example, existing SSLs can have a positive temperature coefficient (“PTC”) thermistor spaced laterally apart from the SSEs within the SSL package. Such laterally positioned PTC thermistors are connected to the SSEs by wirebonds. When the temperature of the SSL package increases beyond a certain threshold, the resistance of the PTC thermistor increases, which in turn reduces the current to the SSEs to limit the light output and heat. In existing passive temperature control systems, a single PTC thermistor can control the power to a plurality of separate SSEs in a multi-chip SSL.
One challenge of existing passive temperature control systems is that they have relatively slow response times because the temperature of the laterally positioned PTC thermistor lags the temperature of the SSEs. Because of such slow response times, the SSL packages can overshoot the actual maximum desired temperature before the PTC resistance has increased sufficiently to limit the light output at high temperatures, or the light output is depressed for too long before the PTC resistance decreases to increase the light output at lower temperatures. Another challenge is that the laterally spaced PTC thermistor reduces the light output per unit area because the PTC thermistors absorb some of the light and occupy space that could otherwise be occupied by an SSE. Still another drawback of existing passive temperature control systems with a single PTC thermistor that modulates a plurality of SSEs is that individual SSEs may not operate at an optimal temperature. In many multi-chip SSLs, some of the SSEs may run hotter or colder than others. The single PTC thermistor cannot compensate for such differences, and thus some of the SSEs may degrade or fail sooner than others.